Static trip circuit breakers which rely on electronic rather than thermal and electromagnetic current responsive trip unit elements are readily available in complete analogue implementation as well as in digital implementation in part. The digital implementation of the trip unit circuitry generally requires a digital processor for providing at least the time delay determination by calculating I.sup.2 t or by referring to stored values. U.S. Pat. No. 4,266,259 to E. K. Howell describes an analogue static trip unit which utilizes some circuit elements that are not easily implemented by integrated circuit techniques. U.S. Pat. No. 4,038,695 to R. P. DePuy utilizes an A/D converter and a binary counter to provide the time delay while performing the overcurrent "pickup" condition in a manner similar to that described by Howell.
U.S. Pat. Nos. 4,423,459; 4,347,541 and 4,442,472 each discloses variations of analogue and digital implementation of both overcurrent pickup and time delay within a single static trip unit circuit. All of the aforementioned patents are incorporated herein for purposes of reference.
Microprocessor-based static trip units are commercially available which utilize an A/D converter along with volatile and non-volatile memory elements for storing time over current data and for determining time delay by means of programs stored within the non-volatile memory elements. One such microprocessor-based static trip unit is described within co-pending U.S. patent application Ser. No. 626,341--filed 6/29/84 which application is incorporated herein for reference purposes and should be referred to for a detailed description of both stored time delay and protective relay algorithms.
From a cost standpoint it would be highly desirable to incorporate the entire static trip unit circuit within a single chip to facilitate automated processing of the overall breaker assembly. It is not known at this time whether any of the hybrid analogue and digital circuits described earlier are capable of complete implementation by means of very large scale (VLSI) integrated circuit techniques. Most known static trip units employ analogue comparators to determine overcurrent pickup and utilize digital counters to provide the requisite time delays. This combination of analogue and digital circuit elements is not readily implemented within a single integrated circuit chip.
The present invention is capable of complete integration by V.L.S.I. implementation by providing the current squaring function, the overcurrent pickup function as well as the time delay function by digital circuitry. The complete trip unit of the invention is capable of automated assembly by insertion of a 40 pin circuit chip containing all the trip unit functions within the breaker control module. It is contemplated that the compact size and low cost of the trip unit chip will allow a wide range of commercial breaker frame sizes to utilize the electronics provided by the chip in place of the thermal and electromagnetic trip components now presently being utilized in the smaller frame sizes.